Electromagnetic detection/identification systems for human beings, animals and goods are at present used for countless applications. Such systems are used, inter alia, for permitting authorized persons access to, for example, protected premises. In such a system, one or more transmitters/receivers are disposed at the entrances and exits of a building, which are capable of recognizing a detection label or responder in the interrogation field. Thereafter the system can verify whether the person carrying the responder is authorized to enter or even leave the building or part of a building at that time Electromagnetic detection systems are also used for the identification of articles. The recognition of articles may be of interest for the control of logistic processes in industrial enterprises. For example, these systems are used in the automobile industry for recognizing containers with specific parts and guiding these to the desired production site These detection and identification systems can be so arranged that the responders do not require a power source of their own, but on the contrary, obtain their supply voltage from the interrogation field.
The major disadvantage of presently known systems is that correct identification is only possible if only one responder is present in the interrogation field. If two or more responders are present in the interrogation field, the unique codes generated by the responders are distorted, as a result of which the validation of the distorted codes leads to error detection in the transmitter/receiver. The result is that none of the responders can be detected. The condition that only one single responder may be present in the interrogation field presents problems for a considerable number of applications of detection systems, which necessitate adaptations which ensure that only one responder is present in the interrogation field at any time.
A method of identifying a plurality of responders in one interrogation field is disclosed in European patent application 0285419 and in applicant's Netherlands patent application 88,02718. That method involves an algorithmic selection procedure which uses a signal sent along with the interrogation carrier, which signal causes only one or a group of the responders that are to be identified to answer back with their code(s), and temporarily deactivates the other responders. In this arrangement, a strict sequence is applied for the identification. The disadvantage of this principle is that the operation is highly complex, so that the responder circuit is in practice relatively large, which has an adverse effect on current consumption Furthermore, identifying responders one by one requires a great deal of communication between the transmitter/receiver and the responders. Moreover, this makes the cycle for the identification of the various responders relatively long.